1. Field of the Invention
The present invention relates to an ink-jet printing method, more specifically, relates to an ink-jet printing method for carrying out printing of characters, images, or the like on a recording medium such as printing paper, OHP paper using an ink and a liquid which makes a coloring material in the ink insoluble (hereafter referred to as a treating liquid).
2. Related Background Art
The ink-jet printing method has various advantages including low noise operation, low running cost, high-speed printing, and easy adaptation for down-sizing and for color printing. Thus, ink-jet printing is widely employed for printers, copying machines, or the like. In general, the ink employed for printers is selected in consideration of its printing properties such as ejecting properties and fixing properties and print qualities such as bleeding, optical reflection density, and coloring of the printed images. As is well known, inks are divided into two types according to the coloring material contained in the ink, i.e., the dye ink and the pigment ink.
The pigment ink has various advantages in comparison with the dye ink, such as superior water resistance and light fastness, and clear character printing. On the other hand, in comparison with the dye-based ink, it takes time to fix the pigment-based ink on a recording medium, the rub-off resistance of the fixed images is not sufficient sometimes, and the size of the ink dot formed on the recording medium by one ejection from the nozzle tends to be small. That is, the pigment in a pigment ink is generally stably dispersed in the ink by the electric repulsion force of the polymer dispersant which destroys the agglomeration of the pigment particles due to intermolecular force. Therefore, it is preferable to add a polymer dispersant to an ink in an amount according to the amount of the pigment. When such a pigment-based ink is applied onto a recording medium such as plain paper by an ink-jet recording process for printing characters, the solvent, e.g. water, of the ink penetrates into the paper and evaporates into the ambient air, and the pigment particles agglomerate. At that time, the more a polymer dispersant is added, the stronger the agglomeration force of the ink on the paper becomes. Thus, when an ink dot is formed on a paper sheet with a prescribed volume of the ink ejected from an ink-jet head, the diameter of the dot is small and the dot shape remains irregular due to the impact of landing on the paper. Therefore, in order to obtain an ink dot having a sufficient recording density and a diameter necessary for forming a recording image without white stripes or other similar defects, the ejection volume of an ink from an ink jet head must be adjusted rather high. Even with such an adjustment, the presence of a polymer dispersant in a pigment ink, in combination with the penetrability decrease of the ink due to the strong agglomeration force of the pigment particles adsorbed on the dispersant, may delay the fixation of a pigment ink on a recording medium or lower the rub-off resistance of the recorded image.
In order to make the dot diameter larger and to improve the fixation properties, a penetrant may be added to a pigment ink to enhance penetration of the ink into a recording medium. However, use of a penetrant may cause undesirable phenomena such as the irregular peripheral shape of the dot (feathering), and penetration of the ink to the back of the recording medium (back-through), which are undesirable for the high quality recorded image. Further, since the coloring material penetrates into the recording medium, it often occurs that the optical density (OD) of the ink dot is not so much increased as the dot diameter increases. In business use application of the ink-jet printers, which will be developed intensively, more improvement will be required in printing speed. In that case, insufficient fixation of the ink onto the recording medium may cause following phenomena: when printed recording medium such as paper sheets are continuously output from the ink-jet printer to be piled one after another, the image on a paper sheet may be disturbed or the set off of the image may occur.
To solve such problems, inks containing a self-dispersing pigment have been proposed. Such an ink can provide dots of a larger diameter, since the pigment agglomeration force on the paper is weaker than that in conventional pigment inks in which the pigment is dispersed by a dispersant. It, however, is not sufficient yet.
As mentioned above, there are still much room for study and development in the printing method to satisfy various factors determining the quality of printing such as fixability of the ink, enlargement of the ink dot diameter, uniform density in an ink dot, and high optical density of the ink itself.
Meanwhile, in order to further improve printing quality by the ink-jet printing process such as water fastness and optical density of the print on a recording medium, there has been proposed and practically used a method to provide onto a recording medium an ink and a treating liquid which is reactive with the ink so as to make them react on the recording medium.
In purpose of solving the above-mentioned problems of the pigment ink but still maintaining the advantages of the pigment ink, the inventors of the present invention have been studying the above-mentioned ink-jet printing technology using a pigment-based ink and a treating liquid which breaks the pigment dispersion state of the ink by reaction. As a part of the study, the inventors have carried out a recording process, in which a pigment ink is applied after a treating liquid was applied onto the recording medium so as to be mixed with the pigment ink in a liquid state. The quality of the resultant image was not necessarily satisfactory and in some cases the quality was rather inferior to that formed using the pigment ink only. Specifically, when a pigment ink containing a pigment dispersed in an aqueous medium by a polymer dispersant was used in combination with a treating liquid reactive with the pigment ink, sometimes OD reduction due to the low area factor of the obtained ink dot was observed. The reason why such a phenomenon occurs is not clear, but probably because the agglomeration of the pigment of the ink on the recording medium is promoted to a large extent by the treating liquid. Thus, to increase the optical density, the area factor may be increased by increasing the amount of the ink ejection, but sometimes leading to the inferior fixability. Also, when a combination of a pigment ink containing a self-dispersing pigment and a treating liquid reactive with the ink is used, a phenomenon called xe2x80x9coozingxe2x80x9d or xe2x80x9chazexe2x80x9d is sometimes observed at the peripheral part of the ink dot formed on a recording medium interfering with clear dot formation. FIG. 1 is a schematic plan view of a dot to which this oozing or haze phenomenon has occurred, where a haze part 7 is present due to the oozing around a reaction area of a pigment ink 7 and a treating liquid 6. FIGS. 2A, 2B, and 2C illustrate the assumed mechanism of occurrence of this phenomenon.
When a treating liquid S is applied to a recording medium P (especially plain paper) (FIG. 2A) and then a pigment ink Ip containing a self-dispersing pigment but not containing a polymer dispersant (hereinafter referred to as xe2x80x9cdispersant-free pigment inkxe2x80x9d) is applied thereon as shown in FIG. 2B, a reaction product 9 starts to be formed. As the reaction proceeds, radial xe2x80x9coozingxe2x80x9d occurs from an approximately circular dot of the reaction product as illustrated in FIG. 2C surrounding the dot with xe2x80x9chazexe2x80x9d. Such xe2x80x9coozingxe2x80x9d or xe2x80x9chazexe2x80x9d is recognized as the same as the known feathering in appearance, deteriorating the printing quality.
Above described xe2x80x9coozingxe2x80x9d or xe2x80x9chazexe2x80x9d is assumed to be a chemical reaction or, in a micro-scale, a following phenomenon. A dispersant-free pigment ink reacts with the treating liquid at a relatively high reaction rate, so that immediately the dispersion break down of the dispersed pigment occurs to produce clusters of the reaction product. At the same time, fine particles of the reaction product are also produced, which then flows out into the recording medium with the penetrating treating liquid to cause xe2x80x9coozingxe2x80x9d or xe2x80x9chazexe2x80x9d.
As described above, mere combination of a pigment ink and a treating liquid would cause unexpected phenomena, making it difficult to obtain high quality ink-jet recording images. The inventors has recognized the necessity of the further technical development of an ink-jet recording technology utilizing a treating liquid, overcoming the disadvantages and retaining the advantages of the pigment ink.
Considering the application of the ink-jet printers to the business use, more improvement will be required in printing speed. One of the great technical problems of such high speed printers is the fixability of the ink onto the recording medium. If the fixability is not good, when printed recording media such as paper sheets are continuously output from an ink-jet printer to be piled one after another, the printed image on a paper sheet may be smeared or the setoff of the image may occur deteriorating printing quality or spoiling the appearance of the printed matter.
The present invention is achieved in consideration of the foregoing newly obtained technical knowledge and findings. An object of the present invention is to provide an ink-jet printing method for obtaining a high quality print, using a treating liquid and a pigment ink in ink-jet printing.
Another object of the present invention is to provide an ink-jet printing method which enables rapid ink fixation to a recording medium without spoiling the quality of the printed matter.
According to one embodiment of the present invention, there is provided an ink-jet printing process including a process of recording images on a recording medium, said process comprising the steps of:
(i) applying an ink on a recording medium by employing an ink-jet recording process, and
(ii) applying a treating liquid capable of reacting with the ink;
wherein said ink contains a first pigment and a second pigment dispersed in an aqueous medium, said first pigment being selected from a self-dispersing pigment having at least one anionic group bonded directly or through another atomic group to the surface of the first pigment and a self-dispersing pigment having at least one cationic group bonded directly or through another atomic group to the surface of the first pigment and said second pigment being capable of being dispersed in an aqueous medium by a polymer dispersant,
said ink further containing at least one dispersant selected from the group consisting of polymer dispersants having the same polarity as that of the group bonded to the surface of said first pigment and nonionic polymer dispersants, and wherein said step (i) is conducted subsequently to said step (ii) so as to bring said ink and said treating liquid into contact with each other in a liquid state on said recording medium.
According to another embodiment of the present invention, there is provided an ink-jet printing process comprising the steps of:
(i) applying a first ink,
(ii) applying a second ink, and
(iii) applying a treating liquid reactive with each of said first and second inks in such a manner that the respective first ink, second ink and treating liquid come into contact with on the surface of a recording medium in a liquid state, wherein
said first ink contains a first pigment selected from a self-dispersing pigment having at least one anionic group bonded directly or through another atomic group to the surface of said first pigment and a self-dispersing pigment having at least one cationic group bonded directly or through another atomic group to the surface of the first pigment,
said second ink contains a dispersant and a second pigment capable of being dispersed in an aqueous medium by the polymer dispersant, the dispersant being at least one selected from a nonionic polymer dispersant and a polymer dispersant having the same polarity as that of the group bonded to the surface of the self-dispersing pigment,
said treating liquid containing a compound having a polarity opposite to the group bonded to the surface of the self-dispersing pigment, and
the step (iii) is carried out before the steps (i) and (ii).
By the invention according to respective embodiments as described, high quality images of high OD and high edge sharpness can be obtained and various advantages such as rub-off resistance and enhanced fixability can be obtained.
The reason why such effects can be achieved by the these embodiments is not clear. The inventors, however, have done various experiments to observe following facts.
When an ink containing a first pigment and a second pigment is applied to a region of a recording medium where a treating liquid have been applied to overlap or contact the treating liquid in a liquid state, the ink spreads into the region to form an ink dot of a large diameter.
When the second pigment and the treating liquid reacts, the agglomeration force will be too strong to form a dot of a large diameter. Therefore, the coexistence of the first and the second pigments at the reaction with the treating liquid may moderate the agglomeration between the polymer dispersant and the treating liquid, that is, it is considered that the reaction between the first pigment and the reacting component in the treating liquid may moderate the strong entwining phenomenon between the reactive component (e.g. a polymer compound) in the treating liquid and a polymer dispersant in the ink, and further the strong intermolecular force between the second pigment molecules may be moderated by the presence of the first pigment, which allows horizontal diffusion of the ink.
On the contrary, it is considered that the haze phenomenon observed during the reaction between the treating liquid and the first pigment may be alleviated by the reaction between the polymer compound in the treating liquid and the second pigment, or by the enclosure of the fine particles causing the haze by the reaction product. As a result, in spite of the larger dot diameter, haze phenomenon hardly occurs to provide excellent edge sharpness.
Further, since an ink dot of a larger diameter can be formed with a smaller ink amount as described above, better fixability can be obtained. The fixability is improved in combination with the use of the first pigment which can reduce the amount of the polymer dispersant to be added to the ink.
For the embodiments of the present invention, by using a treating liquid having a higher penetrability to the recording medium, the fixability and the dot diameter are further improved. Probably, the treating liquid having a high penetrability to the recording medium diffuses rapidly to form a kind of an ink-receiving layer on the surface of the recording medium, so that the ink can easily penetrate and diffuse on the surface to form a dot reacting with the treating liquid. This is considered to be the reason why a large dot is formed in a short time.
Further, in the embodiments, it is preferable to use a treating liquid of optimized components according to the types and ratio of the first and second pigments in the ink, in order to further improve the image quality. That is, a self-dispersing pigment probably has a prickly shape having a large number of whisker-like polar groups (anionic groups) around the pigment core as illustrated as 3001 in FIG. 3A. On the other hand, a polymer compound, for example, polyallylamine (PAA) having a large number of cationic groups in one molecule is generally as illustrated as 3003 in FIG. 3B. When such a compound is mixed with a self-dispersing pigment, as illustrated in FIG. 4, a PAA polymer 3003 entwines itself around a molecule of the self-dispersing pigment 3001. Since not every cationic group of PAA, however, can make a match with an anionic group, the reaction product of the self-dispersing pigment and PAA may remain cationic as a whole. Thus, reaction products between the fine pigment particles and PAA have weaker intermolecular force, and are liable to electrically repulse each other, hardly forming a large aggregate. Consequently, these fine particles are considered to cause slight bleeding like haze around the dot. On the other hand, in case of a second pigment dispersed by a polymer dispersant, the dispersing ability of the polymer dispersant having a large number of anionic groups or cationic groups by itself is not completely destroyed by a treating liquid containing a compound having one anionic or cationic group per molecule. Hence, by employing a treating liquid containing both a cationic polymer compound such as PAA and a cationic low molecular weight compound such as benzalkonium chloride 3005 in a prescribed ratio and an ink containing a first pigment having an anionic group on the surface and a second pigment dispersed by an anionic polymer dispersant, the dispersability of the respective pigments in the ink is destroyed on a recording medium without fail to suppress the generation of unreacted cationic groups which will cause the haze phenomenon. As a result, high quality images having high density, no haze and excellent fixation can be formed on a recording medium in a short fixation time.